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Date: Mon, 25 Jul 2011 07:03:23 -0700
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Cc: freebsd-questions@freebsd.org
Subject: Re: ATA troubles
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Jerome Herman <jherman@dichotomia.fr> wrote:

> >> Jul 24 23:48:36 mydavid kernel: ad6: FAILURE - READ_DMA48 
> >> status=51<READY,DSC,ERROR> error=40<UNCORRECTABLE> LBA=1671887488
> >> Jul 24 23:48:36 mydavid kernel: 
> >> g_vfs_done():stripe/backup[READ(offset=1712012836864, 
> >> length=131072)]error = 5
> ... since they are ATA drives make sure you are using 80pins
> ribbons and that DMA is properly activated in BIOS.
>
> You can also try to reduce DMA level, it must be on UDMA5 by
> default, try using UDMA 4 (aka UDMA/66) or UDMA 3.

I fixed a similar problem -- involving a VIA 6421 controller --
a while back, by using atacontrol(8) to reduce the DMA speed
from UDMA133 to UDMA100.  Evidently it is possible, under some
circumstances, for a device and controller to negotiate a speed
that's too high to actually work :(